The present invention concerns safety valves used in the control of pressurized-fluid consumers, especially hydraulically or pneumatically controlled couplings and brakes in presses. Safety valves of the type in question typically comprise a single valve housing accommodating two switching valves acting in parallel and used to control the feeding of pressure fluid (gaseous or liquid) to and from the consumer. The parallel-acting switching valves are for example activated by electromagnetically controlled pilot valves. Finally, such safety valves are provided with a safety device for switching off the consumer in the event of malfunction.
Safety valves of this type are used, for example, in order to be able to reliably shut off a press, in the event that one of the two switching valves fails, or in the event that one of the switching valves or its associated pilot valve does not activate properly. If that occurs, then it is known, for example to employ a piston manometer, or other safety device, which is activated by the pressurized fluid and moved by it out of inoperative position to a switch-activating position activating an emergency switch serving to shut off the electrical equipment of the press.
It is also known to sense the positions and strokes of the pistons of the two power-switching valves using contactless inductive or capacitive proximity detectors. These produce a signal in the event that the pistons of the two switching valves do not perform identical movements, i.e., when both valves are not opening or closing simultaneously. An assumption underlying this known technique is that a mulfunction reflected in the movement or position of one valve piston will not also be reflected in the movement or position of the other.
A disadvantage of these prior-art automatic monitoring techniques is that they do not indicate, for example, the fact that the valve head of one switching valve has broken or that the sealing ring of the valve seat of one of the valves has failed. Also, these techniques are not capable of detecting damage to or leakage from the connecting conduits between the safety valve(s) and the consumer(s).